You will quickly encounter the so-called free spins at Casino Mate. They are part of the welcome bonus you can get when you register as a player at the casino.
Free spins are really located in the name. These are a series of spins that you either get for a predetermined slot machine or for one of your choice. The casino will tell you under the bonus terms.
The spins allow you to start playing without breaking the cash bonus, ie the money you have in your casino account. They thus give you a small head start so you can get started.
If the free spins are for an optional slot machine, this is a really good opportunity for you to try a little different games. Not all players are quite sure which games they like best. Thus, they give you a little leeway to try some slots you might not normally have tried.